The basic principles of DC motors are presented by Joe Wolfe of the University of New South Wales (Sydney, Australia) at the link:
http://www.animations.physics.unsw.edu.au/jw/electricmotors.html

Phyclips from UNSW

Northwestern University (Chicago, Illinois) presents the basics of a DC motor in the Wiki page:
http://hades.mech.northwestern.edu/wiki/index.php/Brushed_DC_Motor_Theory

Northwestern University Wiki page